Method for manufacturing a thin filtering membrane and an acoustic transducer device including the filtering membrane

ABSTRACT

A method for manufacturing a filtering module comprising the steps of: forming a multilayer body comprising a filter layer of semiconductor material and having a thickness of less than 10 μm, a first structural layer coupled to a first side of the filter layer, and a second structural layer coupled to a second side, opposite to the first side, of the filter layer; forming a recess in the first structural layer, which extends throughout its thickness; removing selective portions, exposed through the recess, of the filter layer to form a plurality of openings, which extend throughout the thickness of the filter layer; and completely removing the second structural layer to connect fluidically the first and second sides of the filter layer, thus forming a filtering membrane designed to inhibit passage of contaminating particles.

BACKGROUND Technical Field

The present disclosure relates to a method for manufacturing a filteringmembrane, an acoustic transducer device including the filteringmembrane, a method for assembling the acoustic transducer device, and anelectronic system including the acoustic transducer device.

Description of the Related Art

In a known way, an acoustic transducer (in particular, a microphone) ofa MEMS (Micro-Electro-Mechanical System) type comprises a sensitivemembrane structure designed to transduce sound pressure waves into anelectrical quantity (e.g., a capacitive variation), and a readelectronics designed to perform appropriate operations of processing(amongst which operations of amplification and filtering) of saidelectrical quantity so as to supply an electrical output signal (e.g., avoltage) representing the acoustic pressure wave received. In the casewhere a capacitive sensing principle is used, the microelectromechanicalsensitive structure in general comprises a mobile electrode, obtained asa diaphragm or membrane, arranged facing a fixed electrode, to providethe plates of a sensing capacitor with variable capacitance. The mobileelectrode is anchored by its first portion, which is generallyperimetral, to a structural layer, whereas a second portion of themobile electrode, which is generally central, is free to move or bend inresponse to the pressure exerted by the incident sound pressure waves.The mobile electrode and the fixed electrode thus form a capacitor, andbending of the membrane that constitutes the mobile electrode causes avariation of capacitance as a function of the acoustic signal to bedetected.

Illustrated in FIG. 1 is an acoustic transducer device 19, representedin a triaxial system of co-ordinates x, y, z. The acoustic transducerdevice 19 comprises a first die 21, which integrates a MEMS structure 1,in particular a MEMS acoustic transducer (microphone), provided with amembrane 2, which is mobile and of a conductive material, facing a rigidplate 3 (by this term is here meant an element that is relatively rigidwith respect to the membrane 2, which is instead flexible). The rigidplate 3 includes at least one conductive layer facing the membrane 2 sothat the membrane 2 and the rigid plate 3 form facing plates of acapacitor.

The membrane 2, which in use undergoes deformation as a function ofincident sound pressure waves, is at least partially suspended over astructural layer 5 and directly faces a cavity 6, obtained by etching arear surface 5 b of the structural layer 5 (the rear portion 5 b isopposite to a front surface 5 a of the structural layer 5 itself,arranged in the proximity of the membrane 2).

The MEMS structure 1 is housed in an internal cavity 8 of a package 20,together with a further die 22, of semiconductor material, whichintegrates a processing circuit, or ASIC (Application-SpecificIntegrated Circuit) 22′. The ASIC 22′ is electrically coupled to theMEMS structure 1 by an electrical conductor 25′, which connectsrespective pads 26′ of the first and second dice 21, 22. The first andsecond dice 21, 22 are coupled side-by-side on a substrate 23 of thepackage 20. The first die 21 is coupled to the substrate 23 on the rearsurface 5 b of the structural layer 5, for example by an adhesive layer;likewise, also the second die 22 is coupled to the substrate 23 on arear surface 22 b thereof. Provided on a front surface 22 a of thesecond die 22, opposite to the rear surface 22 b, is the ASIC 22′.

Appropriate metallization layers and vias (not illustrated in detail)are provided in the substrate 23 for routing of the electrical signalstowards the outside of the package 20. Further electrical connections25″, obtained with the wire-bonding technique, are provided between pads26″ of the second die 22 and respective pads 26″ of the substrate 23.

Further coupled to the substrate 23 is a covering 27 of the package 20,which encloses inside it the first and second dice 21, 22. Said covering27 may be of metal or pre-molded plastic.

Electrical-connection elements 29, for example in the form of conductivelands, are provided on the underside of the substrate 23 (the sideexposed outwards), for soldering and electrical connection to aprinted-circuit board.

The substrate 23 further has a through opening, or hole, 28, whicharranges in fluidic communication the cavity 6 of the first die 21 withthe environment external to the package 20. The through opening 28 (inwhat follows referred to as “sound port”) enables entry of a flow of airfrom outside the package 20 and of the sound pressure waves, which, byimpacting on the membrane 2, cause deflection thereof.

In a known way, the sensitivity of the acoustic transducer depends uponthe mechanical characteristics of the membrane 2 of themicroelectromechanical structure 1 and further upon assembly of themembrane 2 and of the rigid plate 3. In addition, the volume of theacoustic chamber created by the cavity 6 has a direct effect on theacoustic performance, determining the resonance frequency of theacoustic transducer.

There are thus several constraints imposed on assembly of a MEMSacoustic transducer, which render design thereof particularlyproblematical, in particular where extremely compact dimensions arerequired, as for example in the case of portable applications.

In order to protect at least partially the cavity 6 and the membrane 2from dust and/or water and/or other debris that might penetrate throughthe through opening 28, thus reducing the useful dimensions of thecavity 6 and/or forming an electrical leakage path, thus jeopardizingthe performance of the acoustic transducer, it is known to provide afilter (illustrated only schematically in FIG. 1, and designated by thereference 30) outside the package 20 and facing the sound port 28 (at adistance therefrom). Said filter 30 is, for example, coupled to aprotective case of a portable device (e.g., a cellphone) that houses thepackage 20.

In particular, in the case of portable applications, the package 20 ishoused within the protective case of the portable device itself, in sucha way that the sound port 28 in turn faces a respective through opening,or hole, made through the protective case of the portable device viainterposition of the filter 30 itself. The filters currently used aremounted manually on the protective case of the portable device andconsequently present excessive dimensions with respect to the realoperating need, which is that of protecting exclusively the cavity 6, aswell as, obviously, the membrane 2 and the rigid plate 3.

Furthermore, the filter 30 prevents entry of contaminating particlesthrough the hole made through the protective case of the portabledevice, but does not solve the problem of contamination deriving fromparticles of dust or other debris coming from different sources (e.g.,on account of a non-perfect hermetic closing of the protective case).

Likewise known, as described in the document EP3065416, which alsopublished as U.S. Pat. No. 9,769,554, is the use of a filter of siliconintegrated in the microelectromechanical structure 1 and above the hole28, or a filtering module arranged between the cavity 6 and the hole 28,for example comprising a weave of wires that forms a mesh such as todefine through openings of maximum size, measured along the axis xand/or the axis y, comprised between 5 and 40 μm. The latter solutionsprovides protection from contaminants during intermediate manufacturingand assembly steps, or else during steps of mounting of the package inthe portable device. However, in both cases and in general for filtersof a known type, the thickness of the filter affects the acousticperformance of the acoustic transducer 19, impacting on thesignal-to-noise ratio (SNR). It is thus desirable to provide a filterfor acoustic transducers of small thickness so as to minimize the impacton the signal-to-noise ratio (SNR).

BRIEF SUMMARY

One or more embodiments are directed to a method for manufacturing afiltering membrane, an acoustic transducer device including thefiltering membrane, a method for assembling the acoustic transducerdevice, and an electronic system including the acoustic transducerdevice that will enable the drawbacks of the prior art to be overcome.

According to the present invention a method for manufacturing afiltering membrane, an acoustic transducer device including thefiltering membrane, a method for assembling the acoustic transducerdevice, and an electronic system including the acoustic transducerdevice are provided. In particular, the filtering membrane has a limitedthickness so as to present a negligible impact on the acousticperformance of the acoustic transducer device.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For a better understanding of the present invention, preferredembodiments thereof are now described, purely by way of non-limitingexample, with reference to the attached drawings, wherein:

FIG. 1 is a schematic cross-sectional view of an acoustic transducerdevice including a MEMS acoustic transducer with the correspondingpackage, according to an embodiment of a known type;

FIG. 2 is a schematic cross-sectional view of an acoustic transducerdevice including a MEMS acoustic transducer with the correspondingpackage and a filtering module, according to an embodiment of thepresent invention;

FIGS. 3A and 3B are top plan views of a portion of the acoustictransducer device of FIG. 2, according to respective embodiments;

FIG. 4 is a schematic cross-sectional view of an acoustic transducerdevice including a MEMS acoustic transducer with the correspondingpackage and a filtering module, according to another embodiment of thepresent invention;

FIG. 5 is a schematic cross-sectional view of an acoustic transducerdevice including a MEMS acoustic transducer with the correspondingpackage and a filtering module, according to a further embodiment of thepresent invention;

FIGS. 6A-6E show a method for manufacturing a filtering module common tothe embodiments of the acoustic transducer devices of FIGS. 2, 4, and 5;

FIGS. 7A-7G show a method for manufacturing a filtering module common tothe embodiments of the acoustic transducer devices of FIGS. 2, 4, and 5and alternative to the manufacturing method of FIGS. 6A-6E;

FIGS. 8A-8F show a method for manufacturing a filtering module common tothe embodiments of the acoustic transducer devices of FIGS. 2, 4, and 5and alternative to the manufacturing method of FIGS. 6A-6E; and

FIG. 9 shows an electronic system including an acoustic transducerdevice according to any one of the embodiments of FIGS. 2, 4, and 5.

DETAILED DESCRIPTION

FIG. 2 shows, in cross-sectional view, an acoustic transducer device 51according to an aspect of the present invention. Elements that are incommon with the acoustic transducer device 51 of FIG. 2 and the acoustictransducer device 19 of FIG. 1 are designated by the same referencenumbers and are not described any further.

In greater detail, the acoustic transducer device 51 of FIG. 2 includesa package 50, formed by the base substrate 23 and by the coveringelement 27. The latter has a substantially cup-like conformation, and iscoupled to the base substrate 23 to form the cavity, or inner space, 8of the package 50. Provided right through the thickness of the basesubstrate 23 is the through opening 28, designed to arrange in acousticcommunication the cavity 6 of the first die 21 with the environmentexternal to the package 50. In what follows, the through opening 28 willalso be referred to as “sound port”, and the cavity 6 of the first die21 will also be referred to as “acoustic chamber”. Further, the term“acoustic communication” is here used with the meaning of “directacoustic communication”, in the sense that generic sound waves, oracoustic pressure waves, propagate in the environment considered usingas sole propagation medium air (or a possible gas, or mixture of gases,equivalent from the standpoint of acoustic propagation).

The extension (in the horizontal plane xy) of the acoustic chamber 6 isgreater than the corresponding extension (once again in the horizontalplane xy) of the sound port 28 in such a way that the sound port 28communicates entirely with the acoustic chamber 6 without having adirect outlet into the inner space 8 of the package 50.

According to an aspect of the present disclosure, the acoustic chamber 6of the first die 21 is in acoustic communication with the sound port 28exclusively through a filtering module 52, which is arranged between thesound port 28 and the acoustic chamber 6 of the first die 21. Forinstance, the filtering module 52 may be arranged inside the acousticchamber 6. The filtering module 52 comprises a supporting layer 54 and afiltering layer 56, which is arranged on the supporting layer 54. Theextension (in the horizontal plane xy) of the supporting layer 54 isgreater than the corresponding extension (once again in the horizontalplane xy) of the sound port 28 in such a way that the sound port 28 isentirely surrounded by the supporting layer 54.

Thus, the filtering module 52 creates an obstacle to the passage of dustand/or contaminating particles coming from the environment external tothe package 50, towards the acoustic chamber 6.

According to one aspect of the present invention, the filtering layer 56has a thickness comprised, for example, between 1 μm and 10 μm, inparticular 5 μm, and has a plurality of through openings such that theflow of sound waves is not interrupted or sensibly degraded by thepresence of the filtering module 52. The impact on the signal-to-noiseratio of the acoustic transducer device 51 is thus minimized, and theacoustic communication of the acoustic chamber 6 towards the outside ofthe package 50 is provided. The supporting layer 54 has a thicknesscomprised between 30 μm and 120 μm, in particular 50 μm. Consequently,the filtering module 52 has a total thickness comprised between 51 μmand 130 μm, in particular 55 μm.

The filtering module 52 may be of any material compatible with theprocesses for manufacturing semiconductor devices. In particular, thesupporting layer 54 may be of metal material, insulating material (e.g.,silicon dioxide or silicon nitride), or semiconductor material (e.g.,silicon, polysilicon, or epitaxial silicon). Further, as described ingreater detail hereinafter, the filtering layer 56 may be of polysilicongrown epitaxially or any other method of growing polysilicon.Alternatively, the filtering layer 56 may be of any other materialcompatible with the processes for manufacturing semiconductor devicesthat may be deposited for a thickness of a few micrometers.

The base substrate 23 is, for example, constituted by a multi-layeredstructure, made up of one or more layers of conductive material(generally metal) separated by one or more dielectric layers (e.g., of aBT—Bismaleimide Triazine—laminate). Electrical paths 49 are providedthrough the base substrate 23 for connecting an inner surface 23 athereof, facing the inner space 8, to an outer surface 23 b thereof,facing the external environment, which supports theelectrical-connection elements 29. The latter are provided, inparticular, in the form of lands, in the case of so-called LGA(Land-Grid Array) packages, as illustrated in FIG. 2. Alternatively, thepads 29 may be replaced by an array of balls or bumps, obtaining aso-called BGA (Ball-Grid Array) package.

According to a different embodiment, the base substrate 23 does notcomprise metal layers or layers of conductive material in general and ismade, for example, of plastic material.

Also the covering element 27 may be constituted by a multilayer, forexample including one or more plastic and/or metal layers, and mayadvantageously present a metal coating (not illustrated) on an innersurface 27 a thereof, facing the inner space 8, in order to provide anelectromagnetic shield. Alternatively, the covering element 27 iscompletely of metal.

The covering element 27 is further coupled to the base substrate 23 soas to seal the inner space 8 hermetically.

FIG. 3A is a schematic top plan view of a portion of the acoustictransducer device 51. In particular, FIG. 3A presents the arrangement ofthe filtering module 52 with respect to the structural layer 5. Thesupporting layer 54 and the filtering layer 56 have a parallelepipedalshape. Further, the supporting layer 54 has a cavity, for example withpolygonal cross-section, which extends throughout the thickness of thesupporting layer 54. Further, as has been said, the filtering layer 56has a plurality of through openings 58, for example with circularsection having a diameter comprised between 1 μm and 10 μm, inparticular 5 μm. In other embodiments, the through openings 58 may havea polygonal cross-section of dimensions such that a circumferenceinscribed in the polygon will have a diameter comprised in the samerange specified for the aforementioned circular section. Consequently,the filtering layer 56 prevents passage of contaminating particleshaving a size larger than the aforesaid diameter.

The through openings 58 may be arranged in an array configuration, inwhich through openings 58 adjacent to one another are at a distance fromone another (measured in the horizontal plane xy between the respectivecentroids) comprised between 3 μm and 15 μm, in particular 7 μm. Inother embodiments, the through openings 58 may be arranged in anirregular way.

The through openings 58 may extend in a central region of the filteringlayer 56, substantially aligned in top plan view to the through opening28, and/or in a peripheral region of the filtering layer 56, arranged intop plan view between the through opening 28 and the supporting layer54.

In general, the number of through openings 58 is selected so as tomaximize the ratio between the sum of the areas (measured in thehorizontal plane xy) of the through openings 58 and the area of thesuspended portion of the filtering layer 56 (measured in the horizontalplane xy limitedly to the regions of the filtering layer 56, whichextend around the through openings 58, i.e., the solid parts of thefiltering layer 56). For instance, the ratio between the sum of theareas of the through openings 58 and the area of the suspended portionof the filtering layer 56 is comprised between 0.3 and 0.7, inparticular 0.45. Said ratio coincides with the ratio between the sum ofthe volumes of the through openings 58 and the volume of the remainingsuspended portions of the filtering layer 56.

FIG. 3B is a schematic top plan view of a portion of the acoustictransducer device 51 a according to an embodiment of the presentinvention alternative to that of FIG. 3A.

In the acoustic transducer device 51 a of FIG. 3B, the supporting layer54 is shaped like a prism with a polygonal base, for example octagonal,and has a cavity with circular section, which extends throughout thethickness of the supporting layer 54. The filtering layer 56 also hasthe shape of a prism with a polygonal base, for example octagonal.

In a way not illustrated in the figure, a first bonding layer extendsbetween the inner surface 23 a of the base substrate 23 and the firstdie 21, and a second bonding layer extends between the inner surface 23a of the base substrate 23 and the filtering module 52. In oneembodiment, the first and second bonding layers coincide and form asingle bonding layer, obtained, for example, by applying preferablynon-conductive glue. A further respective bonding layer (made, forexample, of preferably non-conductive glue or biadhesive tape) extends,in a way not illustrated in the figure, between the inner surface 23 aof the base substrate 23 and the second die 22.

FIG. 4 shows, in cross-sectional view, an acoustic transducer device 61according to a further embodiment of the present invention. Elementsthat are in common to the acoustic transducer device 61 of FIG. 4 and tothe acoustic transducer device 51 of FIG. 2 are designated by the samereference numbers, and are not described any further. In greater detail,in the acoustic transducer device 61 of FIG. 4 the base substrate 23 hasa recess 24, on its inner surface 23 a, having an extension (measured inthe horizontal plane xy) greater than that of the sound port 28 andsmaller than that of the acoustic chamber 6 so as to surround entirely,in top plan view, the sound port 28, and being surrounded entirely, intop plan view, by the acoustic chamber 6. The recess 24 extends from theinner surface 23 a towards the outer surface 23 b of the base support 23for a distance (measured along the axis z) for example comprised between10 μm and 100 μm, in particular 40 μm.

In the acoustic transducer device 61, the filtering module 52 is housedat least partially by the recess 24. In this way, the overall thicknessof the filtering module 52 is only partially constrained by thethickness of the acoustic cavity 6.

In a way not illustrated in the figure, a first bonding layer extendsbetween the inner surface 23 a of the base substrate 23 and the firstdie 21, and a second bonding layer extends between the inner surface 23a of the base substrate 23 in the recess 24 and the filtering module 52.The first and second bonding layers are obtained, for example, byapplying preferably non-conductive glue or, alternatively, a biadhesivetape. A further respective bonding layer (which is also, for example,preferably of non-conductive glue or biadhesive tape) extends, in a waynot illustrated in the figure, between the inner surface 23 a of thebase substrate 23 and the second die 22.

FIG. 5 shows, in cross-sectional view, an acoustic transducer device 71according to a further embodiment of the present invention. In theacoustic transducer device 71 of FIG. 5, the filtering module 52 isarranged on the inner surface 23 a of the base substrate 23 in such away that the cavity of the supporting layer 54 surrounds entirely, intop plan view, the sound port 28. Further, the first die 21 is arrangedon the filtering module 52 to form a stack. Also in this embodiment, theacoustic chamber 6 is in fluid connection with the sound port 28 via thethrough openings of the filtering layer 56, and in particular in such away that the sound waves entering from the sound port 28 present apreferential passage, towards the acoustic chamber 6, through thethrough openings of the filtering layer 56.

In a way not illustrated in the figure, a first bonding layer extendsbetween the inner surface 23 a of the base substrate 23 and thefiltering module 52, and a second bonding layer extends between thefirst die 21 and peripheral regions of the filtering layer 56, at adistance from the through openings 58. The first and second bondinglayers are obtained, for example, by applying preferably non-conductiveglue. A further respective bonding layer (which is also, for example,preferably of non-conductive glue or biadhesive tape) extends, in a waynot illustrated in the figure, between the inner surface 23 a of thebase substrate 23 and the second die 22.

In an embodiment, the thickness of the package 50 of the acoustictransducer device 71 may be greater than that of the acoustic transducerdevice 51 of FIG. 2 and of the acoustic transducer device 61 of FIG. 4,on the basis of the additional thickness of the filtering module 52. Inanother embodiment, in which the thickness of the supporting layer 54 isminimized (e.g., using a manufacturing process described in detailhereinafter with reference to FIGS. 7A-7G), the impact of the presenceof the filtering module 52 on the choice of the thickness of the package50 is negligible.

The acoustic transducer device 71 of FIG. 5 prevents degradation of theacoustic performance of the microphone 1 that might present in theacoustic transducer device 51 of FIG. 2 and in the acoustic transducerdevice 61 of FIG. 4, since the vacuum regions in the filtering layer 56here have a more extensive total area, and associated to this is a lowergeneration of noise.

FIGS. 6A-6E show a process for manufacturing the filtering module 52,common to the acoustic transducer device 51 of FIG. 2, to the acoustictransducer device 61 of FIG. 4, and to the acoustic transducer device 71of FIG. 5.

With reference to FIG. 6A, a wafer 100 is provided, which includes asubstrate 101 of semiconductor material, such as silicon, which extendsbetween a front surface 100 a and a rear surface 100 b of the wafer 100for a thickness comprised, for example, between 500 μm and 1000 μm. Anoxide layer 102 is formed on the front surface 100 a of the wafer 100,having the function of etching mask for subsequent steps of themanufacturing process. In particular, the oxide layer 102 has aplurality of openings 102′ in the regions of the wafer 100 in which thethrough openings 58 of the filtering layer 56 will be formed, in orderto provide through openings 58 that have a desired shape, as discussedpreviously. The oxide layer 102 consists, for example, of silicon oxide(SiO₂) having a thickness comprised between 0.5 μm and 3 μm. The oxidelayer 102 is formed, for example, by a step of deposition of siliconoxide and/or thermal growth of silicon oxide. The openings 102′ of theoxide layer 102 are formed using known photolithographic techniques.

With reference to FIG. 6B, a structural layer 104 is formed on the frontsurface 100 a of the wafer 100, having a thickness greater than thethickness of the oxide layer 102 so as to extend between the openings ofthe oxide layer 102 and cover the oxide layer 102 entirely. In detail,the thickness of the structural layer 104 is the same as the thicknessdesired for the filtering layer, and is thus for example comprisedbetween 1 μm and 10 μm. The structural layer 104 is of semiconductormaterial such as silicon or polysilicon grown, which may includeepitaxially grown, on the substrate 101.

With reference to FIG. 6C, a carrier wafer 110 is provided. The carrierwafer 110 is, for example, of semiconductor material, such as silicon,having a thickness comprised, for instance, between 500 μm and 1000 μm.The carrier wafer 110 is mechanically bonded to the front surface 100 aof the wafer 100 via a bonding layer 106 using wafer-to-wafer bondingtechniques of a per se known type. For instance, a direct bondingtechnique may be used, or else a glass-frit technique, in the case wherethe bonding layer 106 has a silicon-dioxide base. In other embodiments,the bonding layer 106 may comprise metal alloys (e.g., Al/Ge) or elsebiadhesive layers.

The thickness of the substrate 101 is reduced down to a thicknesscomprised between 30 μm and 120 μm, in particular 50 μm, via grinding ofthe rear surface 100 b of the wafer 100.

A masked etch of the wafer 100 is carried out (FIG. 6D), on its rearsurface 100 b, using a photolithographic mask having an opening thatsurrounds entirely, in top plan view, the openings of the oxide layer102.

Masked etching of the wafer 100 is carried out via techniques of surfacemicromachining of a known type, using etching chemistries thatselectively remove the material of the substrate 101 but not thematerials of the oxide layer 102 and of the bonding layer 106, whichfunctions as an etch-stop layer. Consequently, at the end of maskedetching of the wafer 100, through openings are formed in the structurallayer 104, thus forming the through openings 58 of the filtering layer56, as described previously. The thickness of the filtering layer 56 isgiven by the thickness of the structural layer 104 and, where present,of the oxide layer 102. In this embodiment, since the oxide layer 102faces, in use, the sound port 28, it bestows characteristics ofhydrophobicity on the filtering module.

In an alternative embodiment (not illustrated), the oxide layer 102 isremoved so that the thickness of the filtering layer 56 coincides withthe thickness of the structural layer 104. The thickness of thesupporting layer 54 coincides with the thickness of the substrate 101.

In a further embodiment (not illustrated) the oxide layer 102 is removedselectively to form the openings 102′, but is maintained on theremaining surface regions of the substrate 101. In this case, the layer104 is in contact with the substrate 101 at the openings 102′, andextends, for the remaining portions, over the oxide layer 102. Here, thethickness of the supporting layer 54 is equal to the sum of thethickness of the substrate 101 and of the oxide layer 102.

With reference to FIG. 6E, the carrier wafer 110 and the bonding layer106 are removed via chemical etching and/or debonding techniques.

Finally, in a way not illustrated in the figures, a step of sawing orsingulation of the wafer 100 is carried out so as to obtain a pluralityof filtering modules 52.

FIGS. 7A-7G show a manufacturing process common to the acoustictransducer device 51 of FIG. 2, to the acoustic transducer device 61 ofFIG. 4, and to the acoustic transducer device 71 of FIG. 5, alternativeto the manufacturing process of FIGS. 6A-6E.

With reference to FIG. 7A, a wafer 200 is provided having a front side200 a and a back side 200 b. The wafer 200 includes a substrate 201 ofsemiconductor material, such as silicon, which extends between an ownfront surface 201 a and an own rear surface 201 b (here coinciding withthe back side 200 b of the wafer) for a thickness, for example,comprised between 500 μm and 1000 μm. An oxide layer 202 is formed onthe front surface 201 a of the substrate 201, having the function ofetching mask for subsequent steps of the manufacturing process. Theoxide layer 202 is made, for example, of silicon oxide (SiO₂) having athickness comprised between 0.5 μm and 3 μm. The oxide layer 202 isformed, for example, by a step of deposition of silicon oxide and/orthermal growth of silicon oxide.

With reference to FIG. 7B, in the oxide layer 202 a plurality ofopenings 204 are formed in the regions of the wafer 200 in which thethrough openings 58 of the filtering layer 56 will be formed in order toprovide through openings 58, which have a desired shape, as discussedpreviously. Further, in the oxide layer 202 an edge opening 206 isformed in the regions of the wafer 200 in which openings will be formedthat are to separate from one another the filtering modules 52 obtainedfrom the wafer 200 at the end of the manufacturing process. In top planview (not illustrated in the figures) the edge opening 206 surrounds theplurality of openings 204 entirely. The openings of the oxide layer 202are formed by techniques in themselves known of photolithography andchemical etching.

With reference to FIG. 7C, at the front side 200 a of the wafer 200 afirst structural layer 208 is formed, having a thickness greater thanthe thickness of the oxide layer 202 so as to extend between theplurality of openings 204, within the edge opening 206, and so as tocover the oxide layer 202 entirely. In detail, the thickness of thefirst structural layer 208 is comprised between 1 μm and 10 μm. Thefirst structural layer 208 is made of semiconductor material such assilicon or polysilicon grown on the substrate 201. The first structurallayer 208 forms, in subsequent steps of the manufacturing process, thefiltering layer 56.

An etch-stop layer 210 is formed on the first structural layer 208. Theetch-stop layer 210 is patterned in such a way that, in plan view in theplane xy, it is superimposed on the plurality of openings 204 but not onthe edge opening 206.

The etch-stop layer 210 is made, for example, of tetraethylorthosilicate (TEOS) and has a thickness comprised between 0.5 μm and 3μm.

With reference to FIG. 7D, a second structural layer 212 is formed atthe front side 200 a of the wafer 200, having a greater thickness thanthe etch-stop layer 210, so as to extend on the first structural layer208 and on the etch-stop layer 210, entirely covering the etch-stoplayer 210. In detail, the thickness of the second structural layer 212is comprised between 10 μm and 60 μm, in particular 20 μm. The secondstructural layer 212 is made of semiconductor material such as siliconor polysilicon grown, such as epitaxially or any other ways, on thefirst structural layer 208. The first structural layer 208 forms, insubsequent steps of the manufacturing process, the filtering layer 56.

A masked etching of the second structural layer 212 is carried out,using a mask having an opening with extension in the plane xysubstantially coinciding with that of the etch-stop layer 210. Etchingof the second structural layer 212 is performed via techniques ofsurface micromachining of a known type, using etching chemistries thatselectively remove the material of the second structural layer 212 untilthe etch-stop layer 210 is exposed.

With reference to FIG. 7E, a chemical etch of the etch-stop layer 210 isperformed. Chemical etching of the etch-stop layer 210 is, for example,of a wet type, in particular based upon the use of hydrofluoric acid(HF), and is selective in regard to the first and second structurallayers 208, 212. At the end of the etching step, the etch-stop layer 210is completely removed.

With reference to FIG. 7F, the wafer 200 is placed, at its front side200 a, on an adhesive tape (or film) 214. The thickness of the substrate201 is reduced to a thickness comprised between 400 μm and 725 μm, viagrinding on the back side 200 b of the wafer 200.

With reference to FIG. 7G, a non-masked chemical etch is carried out onthe back side 200 b of the wafer 200, entirely removing the substrate201 and selective portions of the first and second structural layers208, 212 not protected by the oxide layer 202. In this way, there areobtained the through openings 58 of the filtering layer 56 andsingulation of the wafer 200 so as to obtain a plurality of filteringmodules 52. In particular, regions of the first structural layer 208separated by the singulation step form respective filtering layers 56 ofrespective filtering modules 52. Further, regions of the secondstructural layer 212 separated by the singulation step form respectivesupporting layers 54 of respective filtering modules 52. Each filteringmodule 52 is detached from the adhesive tape 214 and assembled on thefirst die 21.

The manufacturing process of FIGS. 7A-7G makes it possible to obtain asmaller thickness of the supporting layer 54, and consequently of thefiltering module 52, as compared to the manufacturing process of FIGS.6A-6E, since said thickness is determined by the growth of the layer212, with which thicknesses may be obtained that are much smaller thanthe ones that may be achieved by grinding. The flexibility of mutualarrangement of the filtering module 52 and of the first die 21 thusincreases, favoring, for example, implementation of the acoustictransducer device 51 of FIG. 2, and in particular of the acoustictransducer device 71 of FIG. 5, without any need to increase thethickness of the package 50.

FIGS. 8A-8F show a manufacturing process common to the acoustictransducer device 51 of FIG. 2, to the acoustic transducer device 61 ofFIG. 4, and to the acoustic transducer device 71 of FIG. 5, andalternative to the manufacturing processes described previously.

Following upon the steps already described with reference to FIG. 7A(providing a wafer 300 including a substrate 301 on which asilicon-oxide layer 302 extends), a deposition is carried out (FIG. 8A)of a first structural layer 308 on the silicon-oxide layer 302, toobtain a layer similar to the first structural layer 208 of FIG. 7C.However, in the case of FIG. 8A, the silicon-oxide layer 302 has notbeen previously patterned.

A mask layer 310 (made, for example, of TEOS) is formed, which extendsover the entire surface of the structural layer 308. The mask layer 310is patterned like the oxide layer 202 of FIG. 7B.

With reference to FIG. 8B, a second structural layer 312 is formed in away similar to the first structural layer 212 of FIG. 7C. With referenceto FIG. 8C, a masked etching of the first and second structural layers308, 312 is carried out, using a mask 314 having openings that areentirely superimposed, in plan view in the plane xy, on the openings ofthe mask layer 310. Etching of the first and second structural layers308, 312 is carried out via techniques of surface micromachining of aknown type, using etching chemistries that remove selectively thematerial of the first and second structural layers 308, 312 until thesilicon-oxide layer 302 is reached (FIG. 8D). In this way, trenches areformed, which extend throughout the thickness of the first and secondstructural layers 308, 312, carrying out a singulation of the wafer 300in order to obtain, at the end of the manufacturing process, a pluralityof filtering modules 52. In this way, trenches are further formed, whichextend throughout the thickness of the first structural layer 308 andform, in subsequent steps of the manufacturing process, the throughopenings 58 of the filtering layer 56.

With reference to FIG. 8E, the wafer 300 is placed, at a front side 300a thereof, on an adhesive tape (or film) 316. The thickness of thesubstrate 301 is reduced to a thickness comprised between 400 μm and 725μm, via grinding performed on a back side 300 b of the wafer 300.

With reference to FIG. 8F, a non-masked chemical etch is carried out onthe back side 300 b of the wafer 300, entirely removing the substrate301. The silicon-oxide layer 302 is removed. Each filtering module 52 isdetached from the adhesive tape 316 and assembled on the first die 21.

The manufacturing process of FIGS. 8A-8F makes it possible to obtain asmaller thickness of the supporting layer 54, and consequently of thefiltering module 52, as compared to the manufacturing process of FIGS.6A-6E, since also in this case the final thickness is arranged by thesteps of epitaxial growth or other growth methods of the layer 312, andnot by a grinding step. The flexibility of mutual arrangement of thefiltering module 52 and of the first die 21 thus increases, favoring,for example, implementation of the acoustic transducer device 51 of FIG.2, and in particular of the acoustic transducer device 71 of FIG. 5,without any need to increase the thickness of the package 50. Further,unlike the manufacturing process of FIGS. 7A-7G, the presence of themask layer 310, of silicon oxide, which in use faces the sound port 28,bestows characteristics of hydrophobicity on the filtering module 52.

FIG. 9 shows an electronic system 400 that uses the acoustic transducerdevice 51, 61, 71 according to the respective embodiment of FIGS. 2, 4,and 5.

The electronic system 400 comprises, in addition to the acoustictransducer device 51, 61, 71, a microprocessor (CPU) 401, a memory block402, connected to the microprocessor 401, and an input/output interface403, for example, a keypad and/or a display, which is also connected tothe microprocessor 401.

The acoustic transducer device 51, 61, 71 communicates with themicroprocessor 401, and in particular transduces the electrical signalsprocessed by the ASIC 22′ of the second die 22 associated to the MEMSsensing structures of the first die 21.

The electronic system 400 is, for example, a mobile communicationdevice, a cellphone, a smartphone, a computer, a tablet, a PDA, anotebook, a wearable device, such as a smartwatch, a voice recorder, aplayer of audio files with voice-recording capacity, a console forvideogames, etc.

From an examination of the characteristics of the invention describedand illustrated herein the advantages that it affords are evident.

For instance, minimization of the thickness of the filtering module 52,due to the use of semiconductor layers and to the possibility ofcarrying out singulation of the wafer at the end of the manufacturingprocess (back end of line) means that the impact of the filtering module52 on the signal-to-noise ratio of the acoustic transducer device isnegligible.

Further, the small thickness of the filtering module 52 enablesformation of a stack between the filtering module and the microphone,thus further reducing the impact of the filtering module 52 on theacoustic performance of the acoustic transducer device.

Further, it is possible to integrate the filtering module 52 in theacoustic transducer device with greater flexibility as compared to theprior art.

Finally, it is clear that modifications and variations may be made tothe invention described and illustrated herein, without therebydeparting from the scope thereof.

For instance, the filtering module 52 may have different shapes in planview in the plane xy, for example circular or elliptical or genericallypolygonal, or polygonal with rounded corners.

Further, it is possible to carry out additional steps in themanufacturing process of FIGS. 7A-7G designed to form a layer ofhydrophobic material (e.g., SiO₂) on the surface of the filtering layer56 that faces, in use, the sound port 28.

In addition, with reference to all the embodiments described previously,it is possible to envisage integration of conductive paths on thesurface of the filtering layer 56 that faces, in use, the sound port 28.Appropriate paths for connection to biasing means external to thefiltering module 52 may be integrated in the base substrate 23 and usedfor electrostatically biasing the filtering layer 56 in order to bestowcharacteristics of hydrophobicity on the filtering module 52. For thispurpose, the base substrate 23 may be a substrate of an LGA type,comprising an inner core and one or more metal layers that extend onopposite faces of the core. The core is, for example, defined by a dieof rigid dielectric material, for example FR4.

Further, it is possible to arrange the stack formed by the filteringmodule 52 and by the first die 21 (as represented in FIG. 5) in a recessof the base substrate 23 (of the type illustrated in FIG. 4).

Finally, for each of the embodiments described previously, there may beenvisaged a different configuration of the MEMS acoustic transducerdevice, in particular as regards the geometrical shape of theconstituent elements. In the case where the space inside the package soallows, there may possibly be housed within the package itself also anumber of MEMS sensors in addition to the MEMS acoustic transducer, eachpossibly provided with a sensitive element that requires a communicationtowards the external environment. Further integrated circuits (e.g.,ASICs) may further be provided and housed within a same package.

The various embodiments described above can be combined to providefurther embodiments. These and other changes can be made to theembodiments in light of the above-detailed description. In general, inthe following claims, the terms used should not be construed to limitthe claims to the specific embodiments disclosed in the specificationand the claims, but should be construed to include all possibleembodiments along with the full scope of equivalents to which suchclaims are entitled. Accordingly, the claims are not limited by thedisclosure.

The invention claimed is:
 1. A method for manufacturing a filteringmodule, the method comprising: forming a multilayer body comprising: afilter layer of semiconductor material and having a thickness of lessthan 10 μm, a first structural layer at a first side of the filterlayer, and a second structural layer at a second side of the filterlayer, the second side being opposite the first side; forming a throughopening in the first structural layer; removing selective portions ofthe filter layer to form a plurality of through openings; and removingthe second structural layer to fluidically couple the first and secondsides of the filter layer to thereby form a filtering membraneconfigured to inhibit passage of contaminating particles above athreshold size.
 2. The method according to claim 1, wherein thethreshold size is larger than 5 μm.
 3. The method according to claim 1,wherein removing selective portions comprises forming the plurality ofthrough openings in such a way that a ratio between a sum of the volumesof the through openings and the volume of suspended portions of thefiltering membrane is between 0.3 and 0.7.
 4. The method according toclaim 1, wherein forming the filter layer includes growing asemiconductor layer.
 5. The method according to claim 1, furthercomprising forming a hydrophobic layer at the first side of the filterlayer.
 6. The method according to claim 1, wherein the first structurallayer is a silicon substrate, wherein the filter layer comprises growingpolysilicon on the first structural layer, and wherein forming themultilayer body comprises coupling the second structural layer to thefilter layer.
 7. The method according to claim 6, further comprisingreducing a thickness of the first structural layer.
 8. The methodaccording to claim 5, wherein forming the hydrophobic layer comprisesdepositing hydrophobic material on the first structural layer andpatterning the hydrophobic material to define an etching mask forremoval of selective portions of the filter layer, and wherein removingthe selective portions comprises removing portions of the filter layernot covered by the hydrophobic material.
 9. The method according toclaim 1, wherein the second structural layer is a silicon substrate,wherein forming the filter layer comprises growing polysilicon on thesecond structural layer, and wherein forming the multilayer bodycomprises forming the first structural layer by growing polysilicon onthe filter layer.
 10. The method according to claim 9, wherein growingthe first structural layer continues until a thickness of the firststructural layer between 30 μm and 60 μm is reached.
 11. The methodaccording to claim 1, wherein the second structural layer is a siliconsubstrate provided with an etch-stop region, wherein forming the filterlayer comprises a deposition of polysilicon on the etch-stop region, andwherein forming the multilayer body comprises forming the firststructural layer by growing polysilicon on the filter layer.
 12. Themethod according to claim 11, wherein growing polysilicon on the filterlayer continues until a thickness between 30 μm and 60 μm is reached.13. The method according to claim 5, wherein forming the hydrophobiclayer comprises depositing hydrophobic material on the filter layer andpatterning the hydrophobic material to define an etching mask forremoving the selective portions of the filter layer, and whereinremoving the selective portions comprises removing portions of thefilter layer not covered by the hydrophobic layer until the etch-stopregion is reached.
 14. A method, comprising: manufacturing a filteringmodule, wherein manufacturing comprises: forming a patterned layer on afirst surface of a semiconductor wafer; growing a structural layer onthe first surface of the semiconductor wafer and around the patternedlayer, wherein the structural layer is grown to a thickness of 10microns or less; coupling a support wafer to the structural layer;thinning the semiconductor wafer at a second surface, the second surfacebeing opposite the first surface; forming a through opening in thesemiconductor wafer; forming a plurality of through openings in thestructural layer; and removing the support wafer.
 15. The methodaccording to claim 14, wherein the patterned layer is a patterned oxidelayer.
 16. The method according to claim 14, wherein forming the throughopening comprises etching the semiconductor wafer starting at the secondsurface.
 17. The method according to claim 14, wherein the plurality ofthrough openings in the structural layer form a filtering membraneconfigured to inhibit passage of contaminating particles above athreshold size.
 18. The method according to claim 14, wherein thesemiconductor wafer is silicon and the structural layer is silicon. 19.The method according to claim 15, further comprising removing thepatterned oxide layer from the first surface of the semiconductor wafer.20. A method, comprising: manufacturing a filtering module, whereinmanufacturing comprises: forming a patterned layer on a first surface ofa semiconductor wafer; growing a structural layer on the first surfaceof the semiconductor wafer and over the patterned layer, the structurallayer having a thickness of 10 microns or less; coupling a support waferto the structural layer; thinning the semiconductor wafer at a secondsurface, the second surface being opposite the first surface; forming athrough opening in the semiconductor wafer; forming a plurality ofthrough openings in the structural layer; and removing the supportwafer; and coupling the filtering module to a substrate.
 21. The methodaccording to claim 20, wherein coupling the filtering module to thesubstrate comprises coupling the filtering module in a recess of thesubstrate.
 22. The method according to claim 20, wherein the substrateincludes a through opening, wherein the filtering module covers thethrough opening.
 23. The method according to claim 20, wherein theplurality of through openings in the structural layer form a filteringmembrane configured to inhibit passage of contaminating particles abovea threshold size.